1. Field of the Invention
Aspects of the present invention relate to an apparatus for preventing a fixing unit of an image forming apparatus from being contaminated by a toner image in the leading end of a sheet and a method of using such.
2. Description of the Related Art
In an electro-photographic image forming apparatus, an electrostatic latent image is formed on a photosensitive drum, a visible image is developed by adhering a toner to a developer, the visible image is transferred to a printing sheet, and the printing sheet to which the visible image is transferred passes through a fixing unit so that the transferred image is fixed before the printing sheet is discharged. The fixing unit employed in the electro-photographic image forming apparatus applies heat and pressure to the printing sheet to which the toner image is transferred so that the toner image is fixed to the printing sheet. The fixing unit includes a heating roller and a pressing roller which are installed in the feeding path of the printing sheet.
As illustrated in FIG. 1, a pressing roller 8 and a heating roller 9 are installed in the feeding path T of a printing sheet, such as a sheet of paper, a transparency sheet, etc., to face each other. The printing sheets are fed one by one from a sheet cassette (not shown) to the pressing roller 8 and the heating roller 9 by feeding rollers 10.
Reference numeral 11 indicates a sheet sensor to detect the printing sheet. Reference numeral L1 indicates a feeding distance from the sheet cassette to the sheet sensor 11 and reference numeral L2 indicates a feeding distance from the sheet sensor 11 to the position where the printing sheet enters between the pressing roller 8 and the heating roller 9.
The fixing unit includes a sheet guide member 7, also known as a “claw.” As illustrated in FIG. 2A, the sheet guide member 7 is installed so that the leading end thereof contacts the surface of the heating roller 9 which has a heater 9a, resulting in a smooth discharge of the printing sheet 6, without curling of the printing sheet 6.
When the printing sheet 6 having a toner image 5 passes through between the pressing roller 8 and the heating roller 9, toner sludge sticks to the sheet guide member 7 and the heating roller 9. Since the leading end of the printing sheet 6 contacts the sheet guide member 7 while passing between the heating roller 9 and the pressing roller 8, the sheet guide member 7 becomes more and more contaminated as the toner image density increases.
As printing is performed and the number of printing sheets 6 passing through the fixing unit increases, the fixing unit becomes severely contaminated, a consequence which is difficult to prevent. If a user wishes to prevent the fixing unit from being contaminated, the density of the toner image transferred to the leading end of the printing sheet 6 should be decreased.
When a copying operation is performed using a multifunction printer having a copy function, and a cover of the multifunction printer is not completely closed, for example, due to the carelessness of a user, the leading end of the printing sheet 6 is exposed to light. As a result of this exposure, the toner image of the leading end of the printing sheet 6 is printed in a black color having a considerably high density, which further accelerates the contamination of the fixing unit.
When users neglect the contamination of the fixing unit, the contamination material attached to the sheet guide member 7 solidifies due to the increase in temperature caused during the fixing process. When the solidified contamination material on the sheet guide member 7 claws down the printing sheet 6 to be discharged, an undesired vertical black line is printed on the printing sheet 6 and the printing sheet 6 may get jammed.